Electrophotographic recording members and processes of preparing same



March 22, 1966 ORG/1 N/C LAYER SUBS TRA TE SEAL COA 7' OPTIONAL VOLTAGE D. M. BORNARTH T L 3,241,958 ELECTROPHOTOGRAPHIC RECORDING MEMBERS AND PROCESSES OF PREPARING SAME Filed Nov. 29, 1962 FIG.

PHOTOCONDUCTOI? PART/CL 5 WATER INSOLUBLE, ORGAN/C SOLVENT- INSOLUBLE,

WATER L/J/D PREC'OAT OF HIGH ELECTR/CAL RESIST/WT) EX. H

EX. ZZZ

\ EX. ll? JZE COMP/WA T/VE CURVE A 7/ME SECONDS INVENTOR5 DEN/W5 MICHAEL BORN/IKTH ROMAN BfEK/I United States Patent Q ELECTROPHOTOGRAPHIC RECORDING MEM.

BERS AND PROCESSES OF PREPARING SAME Dennis Michael Bornarth, Palatine, and Roman Berka,

Chicago, Ill., assignors to Addressograph-Multigraph Corporation, a corporation of Delaware Filed Nov. 29, 1962, Ser. No. 241,008 16 Claims. (Cl. 96-1) This invention relates to electrophotographic recording members and to processes of producing such members. More particularly this invention relates to electrophotographic recording members useful for producing positive reproductions and which can also be used advantageously in reversal electrophotographic printing procedures.

In retrieving and making reproductions from a film negative, such as, for example, microfilm, which is enlarged by electrophotographic techniques, a reversal image is produced by projecting light through the nega tive onto a charged photo-conductive recording member to form a latent electrostatic image, commonly referred to as a reversal image. This image is developed by the application of electroscopic developed powder by any of several well known techniques, such as cascade developers, magnetic brush developers, and liquid developers, and the developer powder is thereafter fixed to form the finished copy.

In positive electrophotographic techniques, the latent electrostatic image is produced by exposing to light the charge in the non-image or background areas, thus discharging these areas in proportion to the light intensity. The image areas are not exposed to light and hence retain their original charge. The electroscopic developer powder applied bears a charge of a polarity opposite to that on the image areas and is thus attracted to and held onto these areas.

In the formation of reversal images on the other hand, the charge in the image areas is exposed to light leaving the non-image or background areas with their original charge and electroscopic developer powder is applied having a charge of a polarity the same as that on the nonimage areas. The charged non-image areas thus repel the developer powder onto the image areas, leaving the nonimage or background areas free of the developer powder. In order to properly develop reversal images it is therefore important that the non-image areas retain a high residual charge, after exposure, of the image areas, at least during the development so that the developer powder will be deposited on the image areas leaving the nonimage or background areas free of developer powder.

Known elect-rophotographic recording members have employed precoats between the substrate, usually paper but which can be cloth, film, metal foil or other suitable substrate, and the top coating layer of insulating film forming resin containing the photo-conductor particles embedded therein, to increase the electrical conductivity of the recording member when the charged member is exposed to light. Increasing the conductivity invariably results in an increase in the dark decay properties of the recording members. The rate at which the charge is dissipated in darkness is referred to herein as the dark decay of the recording member. Heretofore known recording members and particularly those having a precoat as hereinabove disclosed, dissipate the charge in the non-image ice areas in the darkness prevailing in the electrophotographic machine where the record-ing member is charged, for example, by a corona charger, exposed (excepting of course the light image projected onto the charge member) and developed, too rapidly to render them satisfactory for use in reversal printing. For example, many of the heretofore known electrophotographic recording members have such a high rate of dark decay that in from 30 to 40 seconds the charge applied at saturation level is substantially completely dissipated.

Electrophotographic copies, and particularly those made by reversal printing, as hereinabove disclosed are frequently use as lithographic masters to produce multiple copies from the electrophotographic copy. To condition the electrophotographic copy for use as a lithographic master it is the practise to apply a so-called wetout solution to the electrophotographic copy to render the non-image areas hydrophilic, the image areas being hydrophobic due to the use of a hydrophobic resin developer powder. For lithographic-master use it is particularly important that the reversal image be of high density to provide a good ink receptive base. Such reversal images of high density cannot be obtained employing heretofore known electrophotographic recording members having a high dark decay as above noted, because the non-image areas during the interval between exposure and development lose too much of their charge.

Attempts to improve the dark decay properties of electrographic recording members by increasing the thickness of the photo-conductive insulating layer, and thus increase the initial electrostatic charge capacity, have proved unsuccessful. Not only is this procedure uneconomical, but it results in a recording member lacking in the necessary flexibility and pliability required for lithographic masters. Reversal images produced on such recording members are poor; the image areas lack the density required to make good lithographic copies and elect-roscopic developer powder adhere to the background areas with consequent production of lithographic copies with poor contrast.

For reversal type photo-conductive members it is particularly important that the surface photo-conductive layer be uniform in composition and be capable of receiving an adequate electrostatic charge say at least 400 volts and desirably from 400 to 800 volts when charged by a corona discharge from a 6200 volt D.C. source. In heretofore known photo-conductive members produced by application of the photo-conductor particles, usually zinc oxide, in .a film forming resin solvent system to a fibrous substrate, usually paper, the penetration of the solvent solution of resin into the fibrous substrate, which penetration frequently varies due to variations in the density or absorbtivity of different areas of the fibrous substrate, results in photoconductive surface layers, having in different areas of one and the same recording member or in different recording members, variable ratios of photoconductor particles to resin and unduly low electrical resistance values with consequent inability to receive an adequate charge.

Other problems encountered with heretofore known electrophotograph-ic recording members employing paper substrates are that when treated with the wet-out solution for use in forming a lithographic master they frequently exhibit curling tendencies of a severity which interferes with their use as lithographic masters, and the images fixed by heat fusion of the developer powder, as conventional, do not adhere adequately to the paper base with consequentl early image failure when employed as a lithographic master.

It is a principal object of the present invention to provide electrophotographic recording members free or substantially free of the above-noted objections of heretofore known recording members.

It is another object of this invention to provide a paper base electrophotographic recording member eminently satisfactory for use in reversal printing to produce copies which can be treated with known wet-out solutions to form lithographic masters capable of producing multiple copies of good quality, the number of such copies being relatively high as compared with the number obtainable from heretofore known electrophotographic paper base recording members.

Still another object of this invention is to provide a process of producing such recording members.

Other objects and advantages of this invention will be apparent from the following detailed description thereof.

The electrophotographic recording members embodying this invention comprise a substrate, which can be any of the known substrates used, such as electrically conductive paper, including high wet strength paper of 3 to 6 mils thickness and having, if desired, a seal coat on its undersurface, cloth, plastic film including cellophane, metallic foils, e.g., aluminum or copper foils, and a precoat on the top face of this substrate, which precoat consists essentially of the reaction product of a water soluble aminoplast precondensate and a film forming polymeric material having at least one reactive carboxyl or hydroxyl group which reaction product is insoluble in water and in the organic solvent, usually toluene, employed for the film forming resin constituent of the top coat. This precoat completely covers the face of the substrate and has a relatively high electrical resistivity, at least 10 ohms/ cm., preferably from 10 to 10 ohms/cm. The photoconductive layer or top coat covers this precoat layer or film. The photo-conductive layer consists essentially of a homogeneous mixture of film forming resin containing the photo-conductor particles which can be any known photo-conductor such as the oxides of zinc, antimony, aluminum, bismuth, cadmium, mercury, molybdenum, and lead; the iodides, selenides, sulfides or tellurides of these metals; selenium; arsenic trisulfide; lead chromate and cadmium arsenide. The preferred photo-conductor is zinc oxide such, for example, as the photo-conductive zinc oxide sold by the New Jersey Zinc Co., as Florence Green Seal 8. I

The preferred pre-coat is the reaction product of watersoluble aminoplast precondensate, such for example as melamine formaldehyde or urea-formaldehyde precondensate and a film forming polymeric material produced by polymerization of ethylenically unsaturated monomer or monomers, which polymeric material contains reactive carboxyl or hydroxyl groups, applied to the substrate in an aqueous medium, and reacted in the proportions of from 1 to 5 parts by weight of polymeric solids to 1 part by weight of aminoplast solids at a temperature of from 250 to 360 F. Examples of polymeric materials which can be reacted with the water-soluble aminoplast precondensate are interpolymers of styrene, hexylacrylate, acrylonitrile and methacrylic acid disclosed in US. Patent 2,767,153, sold by the Monsanto Chemical Co., under their trade name Lytron 680; polyvinyl alcohol; carboxylated styrene maleic anhydride coplymers; carboxylated ethylene maleic anhydride copolymers; hydroxyethyl cellulose; and crotonic acid vinyl acetate copolymers.

The aminoplast precondensate can be any of the watersoluble aminoplasts such as dimethylol melamine, dimethyl trimethylol melamine, trimethylol melamine, tetramethylol melamine, tetramethyl pentamethylol melamine,

tetramethyl hexamethylol melamine, pentamethyl pentamethylol melamine, pentamethyl hexamethylol melamine, other alkylated melamine formaldehyde resins, or mixtures of such resins, urea formaldehyde, aniline formaldehyde, thiourea formaldehyde, and cyanamide formaldehyde.

The recording members are prepared by coating the substrate, preferably in web form, with an aqueous coating composition containing from 60 to by weight of water, having dissolved therein the aminoplast precondensate and the polymeric material, when a water soluble polymeric material is used. When employing a polymeric material in the form of a latex or dispersion, the latex is mixed with the solution of aminoplast precondensate. If desired this coating composition can contain a small amount of a stabilizing agent to prevent premature reaction of the aminoplast and polymeric material. Where a product of relatively high flexibility is desired, a plasticizer or fiexibilizing agent, such as polyvinyl acetate which imparts greater flexibility to the precoat, is also incorporated. The coating composition is applied to the substrate using any known coating equipment such as spray or roll type coaters which apply the coating to one surface of the substrate in amount to provide a layer or film, which when dry weighs about 4 pounds per 3,000 square feet of substrate. When producing an electrophotographic recording member having a seal coat on the back, of the same composition as the pre-coat on the face, the coating composition can be applied employing immersion type coating equipment. The coated substrate is passed through a curing oven, e.g., a hot air drying chamber at a temperature of from 250 to 360 F., preferably about 350 F., for a residence time of from 30 seconds to 5 minutes. Reaction takes place between the aminoplast and the polymeric material forming a continuous uniform fil'm securely bonded to the substrate which film is water-insoluble and insoluble in the organic solvent for the resin or resins employed in the top coat, of high electrical resistivity and surprisingly imparts to the final product improved photo-conductive and dark decay properties.

The top coat is applied to the pre-coat using any known coating equipment. A homogeneous dispersion of the photo-conductor particles in a solution of the resinous film forming vehicle such for example as the silicone resins of the alkyl aryl type (GE. No. 81,182 or Sr-82 or Dow-Corning No. 803) or styrenebutadiene copolymers in an organic solvent, e.'g., toluene or trichloroethylene, the ratio of photo-conductor particles to resin being from 1 to l to 5 to 1. Thus, for example, the top coat may be formed by applying a dispersion containing from 35% to 50% by weight of photoconductor particles, such as zinc oxide, from 10% to 30%, preferably 25% to 30%, by weight of resin and from 35% to 55% by weight of solvent. The coated substrate is passed through a drying oven maintained at a temperature of 200 F. to 250 F. for a residence time of from 30 seconds to 5 minutes, to remove the volatiles, i.e., the solvent; the solvent can be otherwise evaporated. The resultant top coat contains from 35 to 50 parts by weight of the photo-conductor particles such as zinc oxide and from 10 to 30 preferably 25 to 30 parts by weight of resin.

Typical formulations on a weight basis, useful in producing the pre-coat are:

FORMULATION 1 Percent Dimethyl-trimethylol-melamine 5045.0

Water 60.0-75.0

NH OH 0.3-0.6 Latex interpolymer of US. Patent 2,767,-

Water 75 .0- 85 .0

Styrene or ethylene maleic anhydride copolyrners 2.5-7.0

FORMULATION 4 Dimethyl-trimethylol-melamine 1.0-3 .0 Water 75.0-85.0 Crotonic acid vinyl acetate copolymer 2.5-7.0 Polyvinyl acetate 5.04100 The following examples are given to illustrate preferred embodiments of processes for producing recording members embodying this invention. It will be understood this invention is not limited to these examples.

In these examples all percentages are given on a weight basis; temperatures are given in degrees F. In all examples the substrate is a commercial bleached sulfite paper.

Example I The pre-coat formulation is: Percent Dimethyl-trimethylol-melamine resin precondensate (Pa-rez 613) 13.5 Water 70.5 Ammonium hydroxide 0.5 Styrene-hexyl acrylate-acrylonitrile-methacrylic acid (Lytron 680) 16.0

The dimethyl-trimethylol-melamine resin is dissolved in the water. The ammonium hydroxide is then added; it serves as a stabilizing agent to prevent the premature reaction between the resin with the styrene-hexyl aorylateacrylonitrile-methacrylic acid polymeric material, which is thereafter added as a latex or dispersion. The mixture is stirred until it is homogeneous requiring about 5 to 7 minutes. The ratio of aminoplast solids to solid carboxylate inter-polymer in the resultant dispersion is 1 to 1.20; the pre-coat dispersion has a viscosity of 16-20 centipoise at room temperature.

The dispersion is coated on a continuous web of the paper substrate at a rate which gives a final dry coating 'weight of 5 lbs. per 3,000 sq. ft. The coated web is passed through a forced hot air drying chamber maintained at 350 F., the residence time in this oven is about 1 minute. During this time the resin and polymer react and are cured into a continuous uniform film securely bonded to the paper.

Over the pre-coat layer is next applied the photo-conductive layer having the following composition:

Percent Zinc oxide, Florence Green Seal (mfg. by the New Jersey Zinc Co.) 34.5 Toluene 38.8 Silicone resin solution Sr-82 (mfg. by the General Electric Co.) 27.5

The silicone resin is mixed with a portion of the toluene until it is completely dissolved. The remainder of the solvent is added gradually while stirring the batch. Once the zinc oxide is uniformly dispersed and the mixture is homogeneous, which may require from 2-24 hours ball milling, the solution is applied at room temperature to the pro-coated side of the paper to yield a final dry coating weight of 20 lbs. per 3,000 sq. ft. The thus coated paper is passed through a forced hot air drying chamber which is maintained at a temperature in the range of 200 F. to 250 F. in order to remove the volatiles (eifected in 6 about 1 minute) and to lay down a uniform photo-coriductive layer.

Example II The pre-coat formulation is: Percent Dimethyl-trimethylol-melamine 1.6 Water 85.5

Polyvinyl alcohol (Evanol 51-05 mfg. by Du Pont) 6.5 Polyvinyl acetate-copolymer (Darex-Everfiex A mfg. by Dewey and Almy Co.) 6.5

The weight ratio of polyvinyl alcoholthe polymer having the reactive hydroxyl functional groups, as solids, to the weight of solids of aminoplast is 4:1. The polyvinyl alcohol is added under constant stirring to the water heated to a temperature of F. to 200 F. Stirring continues until a clear, homogeneous solution results which is then allowed to cool to room temperature. The polyvinyl acetate copolymer is then added to the cooled solution while agitating the mixture. The completed formulation is stirred for an additional 5 minutes after addition of all of the polyvinyl acetate copolymer and the resultant composition applied to the paper as in Example I.

A photo-conductive layer is then applied having the following composition:

Percent Zinc oxide, Florence Green Seal 36 Trichloroethylene 52 Styrene-butadiene copolymer (Goodyear Pliolite S-5) 12 This photo-conductive top coat is applied over the precoat as in Example I.

Example III The pre-coat formulation is:

Percent Urea formaldehyde (Uformite 467 Rohm & Haas) 2.5 Water 84.0 Carboxylated-copolymer resin of ethylene maleic anhydride (DX84013Monsanto Chemical Co.) 5.2 Polyvinyl acetate (Darex-Everflex A Dewey and Almy Co.) 8.7

To the water is added first the carboxylated-copolymer resin of ethylene maleic anhydride and then the urea formaldehyde and the mixture agitated until a uniform solution is obtained. The polyvinyl acetate copolymer is then added and agitation is continued for 57 minutes to assure adequate dispersion.

This dispersion is applied to the paper, the coated paper cured and the top coat applied thereto, all as in Example I.

Example IV This example differs from Example II in the substitution of hydroxyethyl cellulose (Natrasol 250, Hercules Powder Co.) for the polyvinyl alcohol, employing substantially the same amount by weight of the hydroxyethyl cellulose as the polyvinyl alcohol. The procedure followed in curing the precoat and applying and curing the top coat is the same as in Example I.

Example V This example differs from Example I chiefly in the substitution of the crotonic acid vinyl acetate copolymer (Gelva C3, Shawinigan Chemical Co.) in amount approximately the same as the amount of interpolymer used in Example I. All other conditions are the same as in Example I.

Example VI This example differs from Example I only in the utilization of a formulation for the pre-coat, containing a ratio of solids of aminoplast to the carboxylated interpolymer of approximately 1 to 5; the formulation for the pre-coat used in Example VI contained 5.0% by weight of the aminoplast solids and 24.0% by weight of polymer (Lytron 680) solids. All other conditions are substantially the same.

Example VII This example differs from Example III only in the substitution of a carboxylated copolymer of styrene and maleic anhydride for the copolymer used in Example III. The amount of the respective copolymers are approximately the same in both examples.

In the accompanying drawing, FIGURE 1 is a vertical section through an electrophotographic recording member embodying this invention; this figure shows the thickness of the paper and the coating layers thereon enlarged as compared with their actual thicknesses, for the sake of clarity of illustration; and

FIGURE 2 is a plot showing dark decay curves of the recording members of the examples of this invention, and for comparative purposes two other recording members not embodying the present invention.

In FIGURE 1, is the substrate, preferably paper such as a commercial grade of paper which has good electrical conductivity properties. Thus substrate 10, if desired, has a base seal coat 11 which can be of the same composition as the pre-coat 12 on the opposite side of the substrate 10. The pre-coat 12, as hereinabove disclosed, is a water-laid, water-insoluble, organic-solvent-insoluble reaction product of the aminoplast precondensate with the polymeric material having reactive carboxyl or hydroxyl groups, which layer 12 is of relatively high electrical resistivity, i.e., has a resistivity of at least about 10 and desirably within the range of 10 to 10 ohms/cm. The precoat of Example I has a resistivity of l.52 l0 ohms/cm; and those of Examples II and III have a resistivity of l.27 10 ohms/cm.

Bonded to the pre-coat layer 12 is the organic solvent laid photo-conductive layer 13 consisting essentially of the photo-conductive particles 14 uniformly distributed throughout and embedded in the resin binder which with the photo-conductive particles forms the layer 13.

The pre-coat layer 12 renders the paper or fibrous substrate 10 resistant to solvent penetration, i.e., prevents penetration into the substrate by the toluene solvent employed as the solvent medium for the film forming resin binder of layer 13, thus resulting in the formation of a smooth uniform photo-conductive layer 13. It also acts as a barrier layer to inhibit moisture or chemical migration through the substrate into the photo-conductive layer 13, and thus results in an electrophotographic recording member of improved properties, particularly from the standpoint of ability to receive a charge at a relatively high saturation level and its dark decay properties.

The electrophotographic recording member of this invention has a combination of desirable properties and is unique and outstanding particularly for use as a recording member in reversal techniques and for the production of copies which are employed as lithographic masters.

The more important of these properties are summarized as follows:

(1) The recording members have relatively high saturation levels for electrostatic charges applied thereto by corona discharge at a given voltage. Thus, their saturation level when charged by corona discharge from a 6200 volt D.C. source of a negative potential with respect to ground, is from 600 to 800 volts, as shown in FIG- URE 2.

(2) They exhibit a slow rate of dark decay; hence, under conditions in existing electrophotographic machines they retain an adequate charge to give dense images upon development.

(3) They have exceptionally good electrostatic properties from the standpoint of receiving a charge and dissipating the charge in areas exposed to light to form good images.

(4) When moistened, as, for example, when wet-out solutions are applied to electrophotographic copies made thereon, they exhibit little or no curling tendency and in this respect, as well as in other respects, the resultant electrophotographic copies are eminently satisfactory for use as lithographic masters.

(5) Fused images formed thereon bond firmly to the surface of the recording members; hence, the recording members do not fail when used as lithographic masters in the early stages of use or before a desired number of copies have been produced. For example, in the case of the recording member of Example II a run was made using this recording member as a lithographic master producing a thousand copies with the initial copy and the final copy of equivalent quality.

The dark decay properties of the recording members of the examples are shown in FIGURE 2. In this figure the curves identified by the legends Ex. I, Ex. II, Ex. III, and Ex. IV-VII are the dark decay curves for the recording members of the respective examples. These curves were obtained by measuring the voltage of the respective recording members (1) at saturation level and when charged in all cases with a corona discharge at 6200 volts from a DC. source at negative potential relative to ground, and (2) at periodic intervals thereafter by means of a suitable probe wrich picked up the charge level at the surface of the respective recording members and fed the resulting signals into a calibrated oscilloscope. The voltage levels of the respective recording members were read from the oscilloscope and curves were made from the data thus obtained.

The curve marked comparative curve A is the dark decay curve obtained as hereinabove described on a recording member made with the same paper base as used in the examples having the same top coat and having a pre-coat which differs from that employed in Example II only in the elimination of the dimethylol-trimethylol-rnelamine and the polyvinyl acetate copolymer from the formulation for the precoat of Example II. In other words, the pre-coat of the recording member, the dark decay characteristics of which are shown by curve A, differs from that of Example II in that the precoat consists of polyvinyl alcohol alone (applied in a water solution having a concentration of about 6.5%) and not the reaction product of polyvinyl alcohol and dimethylol-trimethylol-melamine.

The curve marked comparative curve B is a dark decay curve of an electrophotographic recording member made with the same paper substrate, having a photo-conductive layer of the same composition, and produced in the same manner as the examples, differing from the recording members of the examples only in the elimination of the pre-coat. Such electrophotographic recording elements are typical of present available commercial products.

The marked improvement in charge receptivity and dark decay characteristics of the electrophotographic recording members of this invention as compared with heretofore known recording members is evident from a comparison of the curves of FIGURE 2. Note that the saturation level of the recording members of this invention when exposed to a corona discharge connected to a 6200 volt D.C. source of a negative potential with respect to ground, is from 600 to 800 volts whereas the saturation level of the comparative products A and B are appreciably less, to wit, of the order of 400 to 450 volts. More important, note the marked and surprising difference in dark decay properties. In the case of comparative product B, which is typical of currently known commercial electrophotographic recording members having the photo-conductive insulating layer consisting of silicone resin and zinc oxide photo-conductor FORMULATION 2 Dimethyl-trimethylol-melamine 1.0-3 .0 Water 65.0-86.0

Water 75.085.0

Styrene or ethylene maleic anhydride copolymers 2.57.0

FORMULATION 4 Dimethyl-trimethylol-melarnine 1.0-3.0

Water 75.085.0 Crotoni c acid vinyl acetate copolymer 2.5-7.0 Polyvinyl acetate 5.0-l10.0

The following examples are given to illustrate preferred embodiments of processes for producing recording members embodying this invention. It will be understood this invention is not limited to these examples.

In these examples all percentages are given on a weight basis; temperatures are given in degrees F. In all exampics the substrate is a commercial bleached sulfite paper.

Example I The pre-coat formulation is: Percent Dimethyl-trimethylol-melamine resin precondensate (Parez 613) 13.5 Water 70.5 Ammonium hydroxide 0.5 Styrene-hexyl acrylate-acrylonitrile-methacrylic acid (Lytron 680) 16.0

The dimethyl-trimethylol-melamine resin is dissolved in the water. The ammonium hydroxide is then added; it serves as a stabilizing agent to prevent the premature reaction between the resin with the styrene-hexyl aorylateacrylonitrile-methacrylic acid polymeric material, which is thereafter added as a latex or dispersion. The mixture is stirred until it is homogeneous requiring about 5 to 7 minutes. The ratio of aminoplast solids to solid carboxylate inter-polymer in the resultant dispersion is 1 to 1.20; the pre-coat dispersion has a viscosity of 16-20 .centipoise at room temperature.

The dispersion is coated on a continuous web of the paper substrate at a rate which gives a final dry coating weight of 5 lbs. per 3,000 sq. ft. through a forced hot air drying chamber maintained at 35 0 E, the residence time in this oven is about 1 minute. During this time the resin and polymer react and are cured into a continuous uniform film securely bonded to the paper.

Over the pre-coat layer is next applied the photo-conductive layer having the following composition:

Percent Zinc oxide, Florence Green Seal (mfg. by the New Jersey Zinc Co.) 34.5 Toluene 38.8 Silicone resin solution Sr-82 (mfg. by the General Electric Co.) 27.5

The silicone resin is mixed with a portion of the toluene until it is completely dissolved. The remainder of the solvent is added gradually while stirring the batch. Once the zinc oxide is uniformly dispersed and the mixture is homogeneous, which may require from 2-24 hours ball milling, the solution is applied at room temperature to the pre-coated side of the paper to yield a final dry coating weight of 20 lbs. per 3,000 sq. ft. The thus coated paper is passed through a forced hot air drying chamber which is maintained at a temperature in the range of 200 F. to 250 F. in order to remove the volatiles (elfected in The coated web is passed a about 1 minute) and to lay down a uniform photo-con ductive layer.

Example II The pre-coa-t formulation is: Percent Dimethyl-trimethy-lol-melamine 1.6 Water 85.5

Polyvinyl alcohol Evanol 51-05 mfg. by Du Pont) 6.5 Polyvinyl acetate-copolymer (DarexaEverflex A mfg. by Dewey and Almy Co.) 6.5

The weight ratio of polyvinyl alcohol-the polymer having the reactive hydroxyl functional groups, as solids, to the weight of solids of aminoplast is 4:1. The polyvinyl alcohol is added under constant stirring to the water heated to a temperature of F. to 200 F. Stirring continues until a clear, homogeneous solution results which is then allowed to cool to room temperature. The polyvinyl acetate copolymer is then added to the cooled solution while agitating the mixture. The completed formulation is stirred for an additional 5 minutes after addition of all of the polyvinyl acetate copolymer and the resultant composition applied to the paper as in Example I.

A photo-conductive layer is then applied having the following composition:

Percent Zinc oxide, Florence Green Seal 36 Trichloroethylene 52 Styrene-butadiene copolymer (Goodyear Pliolite S-5) 12 This photo-conductive top coat is applied over the precoat as in Example I.

Example III The pre-coat formulation is:

Percent Urea formaldehyde (Uformite 467 Rohm & Haas) 2.5 Water 84.0 Carboxylated-copolymer resin of ethylene maleic anhydride (DX-840-13-Monsanto Chemical Co.) 5.2 Polyvinyl acetate (Darex-Everfiex A Dewey and Almy Co.) 8.7

To the water is added first the carboxylated-copolymer resin of ethylene maleic anhydride and then the urea formaldehyde and the mixture agitated until a uniform solution is obtained. The polyvinyl acetate copolymer is then added and agitation is continued for 57 minutes to assure adequate dispersion.

This dispersion is applied to the paper, the coated paper cured and the top coat applied thereto, all as in Example I.

Example IV This example differs from Example 11 in the substitution of hydroxyethyl cellulose (Natrasol 250, Hercules Powder Co.) for the polyvinyl alcohol, employing substantially the same amount by weight of the hydroxyethyl cellulose as the polyvinyl alcohol. The procedure followed in curing the precoat and applying and curing the top coat is the same as in Example I.

Example V This example differs from Example I chiefly in the substitution of the crotonic acid vinyl acetate copolymer (Gelva C-3, Shawinigan Chemical Co.) in amount approximately the same as the amount of interpolymer used in Example I. All other conditions are the same as in Example 1.

Example VI This example differs from Example I only in the utilization of a formulation for the pre-coat, containing a ratio of solids of aminoplast to the carboxylated interpolymer of approximately 1 to 5; the formulation for the pre-coat used in Example VI contained 5.0% by weight of the aminoplast solids and 24.0% by weight of polymer (Lytron 680) solids. All other conditions are substantially the same.

Example VII This example differs from Example III only in the substitution of a carboxylated copolymer of styrene and maleic anhydride for the copolymer used in Example 111. The amount of the respective copolymers are approximately the same in both examples.

In the accompanying drawing, FIGURE 1 is a vertical section through an electrophotographic recording member embodying this invention; this figure shows the thickness of the paper and the coating layers thereon enlarged as compared with their actual thicknesses, for the sake of clarity of illustration; and

FIGURE 2 is a plot showing dark decay curves of the recording members of the examples of this invention, and for comparative purposes two other recording members not embodying the present invention.

In FIGURE 1, is the substrate, preferably paper such as a commercial grade of paper which has good electrical conductivity properties. Thus substrate 10, if desired, has a base seal coat 11 which can be of the same composition as the pre-coat 12 on the opposite side of the substrate 10. The pre-coat 12, as hereinabove disclosed, is a water-laid, Water-insoluble, organic-solvent-insoluble reaction product of the aminoplast precondensate with the polymeric material having reactive carboxyl or hydroxyl groups, which layer 12 is of relatively high electrical resistivity, i.e., has a resistivity of at least about 10 and desirably within the range of 10 to 10 ohms/cm. The precoat of Example I has a resistivity of 1.52 10 ohms/cm; and those of Examples II and III have a resistivity of 1.27 l0 ohms/cm.

Bonded to the pre-coat layer 12 is the organic solvent laid photo-conductive layer 13 consisting essentially of the photo-conductive particles 14 uniformly distributed throughout and embedded in the resin binder which with the photo-conductive particles forms the layer 13.

The pre-coat layer 12 renders the paper or fibrous substrate 10 resistant to solvent penetration, i.e., prevents penetration into the substrate by the toluene solvent employed as the solvent medium for the film forming resin binder of layer 13, thus resulting in the formation of a smooth uniform photo-conductive layer 13. It also acts as a barrier layer to inhibit moisture or chemical migration through the substrate into the photo-conductive layer 13, and thus results in an electrophotographic recording member of improved properties, particularly from the standpoint of ability to receive a charge at a relatively high saturation level and its dark decay properties.

The electrophotographic recording member of this invention has a combination of desirable properties and is unique and outstanding particularly for use as a recording member in reversal techniques and for the production of copies which are employed as lithographic masters.

The more important of these properties are summarized as follows:

(1) The recording members have relatively high saturation levels for electrostatic charges applied thereto by corona discharge at a given voltage. Thus, their saturation level when charged by corona discharge from a 6200 volt D.C. source of a negative potential with respect to ground, is from 600 to 800 volts, as shown in FIG- URE 2.

(2) They exhibit a slow rate of dark decay; hence, under conditions in existing electrophotographic machines they retain an adequate charge to give dense images upon development.

(3) They have exceptionally good electrostatic properties from the standpoint of receiving a charge and dissipating the charge in areas exposed to light to form good images.

(4) When moistened, as, for example, when wet-out solutions are applied to electrophotographic copies made thereon, they exhibit little or no curling tendency and in this respect, as well as in other respects, the resultant electrophotographic copies are eminently satisfactory for use as lithographic masters.

(5) Fused images formed thereon bond firmly to the surface of the recording members; hence, the recording members do not fail when used as lithographic masters in the early stages of use or before a desired number of copies have been produced. For example, in the case of the recording member of Example II a run was made using this recording member as a lithographic master producing a thousand copies with the initial copy and the final copy of equivalent quality.

The dark decay properties of the recording members of the examples are shown in FIGURE 2. In this figure the curves identified by the legends Ex. I, Ex. II, Ex. III, and Ex. IV-VII are the dark decay curves for the recording members of the respective examples. These curves were obtained by measuring the voltage of the respective recording members (1) at saturation level and when charged in all cases with a corona discharge at 6200 volts from a DC. source at negative potential relative to ground, and (2) at periodic intervals thereafter by means of a suitable probe wrich picked up the charge level at the surface of the respective recording members and fed the resulting signals into a calibrated oscilloscope. The voltage levels of the respective recording members were read from the oscilloscope and curves were made from the data thus obtained.

The curve marked comparative curve A is the dark decay curve obtained as hereinabove described on a recording member made with the same paper base as used in the examples having the same top coat and having a pre-coat which differs from that employed in Example II only in the elimination of the dimethylol-trimethylol-melamine and the polyvinyl acetate copolymer from the formulation for the precoat of Example II. In other words, the pre-coat of the recording member, the dark decay characteristics of which are shown by curve A, differs from that of Example II in that the precoat consists of polyvinyl alcohol alone (applied in a water solution having a concentration of about 6.5%) and not the reaction product of polyvinyl alcohol and dimethylol-trimethylol-rnelamine.

The curve marked comparative curve B is a dark decay curve of an electrophotographic recording member made with the same paper substrate, having a photo-conductive layer of the same composition, and produced in the same manner as the examples, differing from the recording members of the examples only in the elimination of the pre-coat. Such electrophotographic recording elements are typical of present available commercial products.

The marked improvement in charge receptivity and dark decay characteristics of the electrophotographic recording members of this invention as compared with heretofore known recording members is evident from a comparison of the curves of FIGURE 2. Note that the saturation level of the recording members of this inven tion when exposed to a corona discharge connected to a 6200 volt DC. source of a negative potential with respect to ground, is from 600 to 800 volts whereas the saturation level of the comparative products A and B are appreciably less, to wit, of the order of 400 to 450 volts. More important, note the marked and surprising difference in dark decay properties. In the case of comparative product B, Which is typical of currently known commercial electrophotographic recording members having the photo-conductive insulating layer consisting of silicone resin and zinc oxide photo-conductor particles, the recording member loses more than half its charge in seconds and loses almost the entire charge in from about 35 to 40 seconds. The comparative recording member A having a polyvinyl alcohol precoat while superior to product B is markedly inferior to the recording members of this invention in that the recording member A loses in 30 to 40 seconds so much of the original charge that the residual voltage is not adequate for the production of good copies by reversal techniques. In the case of all of the examples of this invention the rate of dark decay is relatively slow and the products retain charges well above 400 volts after 30-40 seconds. For dense images produced by reversal procedures it is important that the charge in the non-image areas be at least 400 volts during development.

The marked improvement effected by the precoat layer 12 of the electrophotographic recording member is further evident from the data given in the table below.

In this table the values under the column heading percent change in surface resistance, is indicative of the capacity of the recording member containing the photoconductive layer to accept a high saturation charge. This column of values shows the differences in surface resistance between the paper base and in each case the precoated paper of the respective examples, and comparative recording member A having a polyvinyl alcohol pre-coat. The values in this column were determined by measuring the electrical resistance according to Standard ASTM Test Method D-257-54T of the paper base before application of the precoat and after application of the precoat. Of course, comparative product B shows no change in the surface resistance since it contained no pre-coat. It is significant that the recording members of the examples show a percent change in surface resistance of from plus 60 to plus 100%. The polyvinyl alcohol pre-coat on the other hand actually results in a decrease in the surface resistance, in other words, the polyvinyl alcohol precoated paper base is approximately 20% more conductive than the uncoated paper.

This table also contains data on the density of the developed electrostatic images produced on the recording members of the examples as well as on the comparative recording members A and B. These density determinations were made using a standard photovoltmeter. The results are truly comparative because the same equipment and techniques Were used in obtaining the values for all recording members, all of which were subjected to charging, exposing, and developing with the same developer powder under the same conditions and in the same equipment. The value 2 in Photovolts units represents standard black; values above 1 are considered excellent for images produced with the developer powder employed in these comparative tests. Zero is the value for white.

It will be noted from the values in the table that the recording members of this invention show a truly surprising improvement in density of the developed images as compared with the recording members A and B.

It will be understood that various modifications can be made in the formulations for the precoat as well as in the formulations for the top coat, in the substrate and seal coat if used, and hence it is intended that all matter contained in the above description or shown in the accompany- 10 ing drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An electrophotographic recording member consisting essentially of a substrate having thereon a coating layer covering and bonded to the face of said substrate, said coating layer consisting essentially of the waterinsoluble reaction product of a water-soluble aminoplast precondensate and a polymeric film-forming material having a reactive group from the group consisting of hydroxyl groups and carboxyl groups, and a top coating layer bonded to said first-mentioned coating layer, said top coating layer consisting essentially of photo-conductor particles uniformly distributed throughout an electrically insulating film-forming resinous binder.

2. An electrophotographic recording member consisting essentially of a paper substrate having thereon a water-laid coating layer covering and bonded to the face of the substrate, said coating layer consisting essentially of a water-insoluble reaction product of a watersoluble amino-plast precondensate and a polymeric film-forming material having a reactive group from the group consisting of hydroxyl and carboxyl groups and a top coating layer bonded to said first mentioned coating layer, said top coating layer consisting essentially of Zinc oxide photoconductor particles uniformly distributed throughout an electrically insulating film-forming resin binder for said zinc oxide particles, said binder bonding the said top coating layer directly to the first-mentioned coating layer.

3. The electrophotographic recording member as defined in claim 2, in which the coating layer bonded to the face of said substrate consists essentially of a reaction product of an aminoplast precondensate and a film-forming polymeric material produced by polymerization of ethylenically unsaturated monomers.

4. The electrophotographic recording member as defined in claim 3, in which the aminoplast precondensate and the polymeric material are reacted in the proportions of from 1 to 5 parts by weight of solid polymeric material per part by weight of solid aminoplast.

5. An electrophotographic recording member consisting essentially of a paper substrate having thereon a waterlaid coating layer covering and bonded to the face of said substrate, said coating layer consisting essentially of the reaction product of dimethyl-trimethylol-melamine with an interpolymer of styrene-hexyl acrylate-acrylonitrile and methacrylic acid in the proportions of approximately 1 part by weight of melamine resin to 1.2 parts by weight of said interpolymer and a top coating layer bonded to said first-mentioned coating layer, said top coating layer consisting essentially of zinc oxide photo-conductor particles uniformly distributed throughout an electrically insulating film-forming resin binder for said zinc oxide particles, said binder bonding the said top coating layer directly to the first-mentioned coating.

6. An electrophotographic recording member consisting essentially of a paper substrate having thereon a waterlaid coating layer covering and bonded to the face of said substrate, said coating layer consisting essentially of the reaction product of dimethyl-trimethylol-melamine with polyvinyl alcohol in the proportions of approximately 1 part by weight of aminoplast to 4 parts by Weight of said polyvinyl alcohol and a top coating layer bonded to said first-mentioned coating layer, said top coating layer consisting essentially of zinc oxide photo-conductor particles in an electrically insulating film-forming resin binder in the proportions of approximately 10 to 30 parts by weight of resin to from 35 to 50 parts by weight of zinc oxide.

7. An electrophotographic recording member consisting essentially of a paper substrate having thereon a waterlaid coating layer covering and bonded to the face of the substrate, said coating layer consisting essentially of the reaction product of urea formaldehyde precondensate with a carboxylated ethylene maleic anhydride copolymer in the proportions of approximately 1 part by weight of urea formaldehyde to 4 parts by weight of said copolymer and a top coating layer bonded to said first-mentioned coating layer, said top coating layer consisting essentially of zinc oxide photo-conductor particles in an electrically insulating film-forming resin binder in the proportions of approximately 10 to 30 parts by Weight of resin to from 35 to 50 parts by weight of zinc oxide.

8. An electrophotographic recording member consisting essentially of a paper substrate having thereon a water-laid coating layer covering and bonded to the face of said substrate, said coating layer consisting essentially of the reaction product of dimethyl-trimethylol-melamine with hydroxyethyl cellulose in the proportions of approximately 1 part by weight of said melamine resin to l to 5 parts by Weight of said hydroxyethyl cellulose and a top coating layer bonded to said first-mentioned coating layer, said top coating layer consisting essentially of zinc oxide photo-conductor particles in a film-forming resin binder in the proportions of approximately to 30 parts by weight of resin to from 35 to 50 parts by weight of zinc oxide.

9. An electrophotographic recording member consisting essentially of a paper substrate having thereon a water-laid coating layer covering and bonded to the face of said substrate, said coating layer consisting essentially of the reaction product of dimethyl-trimethylol-melamine with an interpolymer of styrene-hexyl acrylate-acrylonitrile and methacrylic acid in the proportions of approximately 1 part by weight of said melamine to 1 to 5 parts by weight of said interpolymer and a top coating layer bonded to said first-mentioned coating layer, said top coating layer consisting essentially of zinc oxide photoconductor particles in a film-forming resin binder in the proportions of approximately 10 to 30 par-ts by weight of resin to from 35 to 50 parts by weight of zinc oxide.

10. A process of producing electrophotographic recording members, which comprises coating a substrate with an aqueous coating composition containing a watersoluble aminoplast precondensate and a film-forming polymeric material having a reactive group from the group consisting of hydroxyl and carboxyl groups, heating the thus coated substrate to react said aminoplast precondensate and the polymeric material thus forming a precoat of high electrical resistivity which is water-insoluble bonded to the face of said substrate, thereafter applying to said precoa't a top coat consisting essentially of photoconductor particles, a film-forming resinous binder for said photo-conductor particles and a volatile solvent for said resinous binder, and evaporating to remove the solvent and bond said topcoat to said pre-coat.

11. A process of producing electrophotographic recording members, which comprises coating a paper base with an aqueous coating composition containing dissolved in the aqueous medium an aminoplast precondensate and also containing a film-forming polymeric material produced by polymerization of ethylenically unsaturated monomers, said polymeric material having reactive groups from the group consisting of hydroxyl and carboxyl groups, heating the thus coated paper base to react said aminoplast precondensate and the polymeric material to form a pre-coat of high electrical resistivity which is waterinsoluble, and thereafter applying to said pre-coat a top coat consisting essentially of zinc oxide photo-conductor particles, a film-forming resinous binder for said photoconductor particles and a solvent, and heating the resultant coated material to a temperature of from 200 F. to 250 F. for from 30 seconds to 5 minutes to produce a photo-conductive topcoat bonded to said pre-coat.

12. A process of producing electrophotographic recording members, which comprises coating a paper base with an aqueous coating composition containing dissolved in the aqueous medium a melamine formaldehyde resin precondensate and also containing a film-forming polymeric material produced by polymerization of ethylenically unsaturated monomers, said polymeric material having reactive groups from the group consisting of hydroxyl and carboxyl groups, heating the thus coated substrate to a temperature of from 250 F. to 360 F. for from 30 seconds to 5 minutes to react said resin with the polymeric material to form a pre-coat of high electrical resistivity which is water-insoluble and thereafter applying to the said pre-coat a top coat, consisting essentially of zinc oxidephoto-conductor particles, a filmforming resinous binder for said photo-conductor particles, and a solvent, and heating the resultant coated material to a temperature of from 200 F. to 250 F. for from 30 seconds to 5 minutes to produce a photoconductive top coat bonded to said pre-coat.

13. The process of producing electrophotographic recording members, which comprises coating a web of paper with a coating composition consisting essentially of from 5% to 15% by weight of 'dimethyl-trimethylol melamine, from 10% to 25% by Weight of a styrenehexyl acrylate acrylonitrile, methacrylic acid interpolymer, a small amount of ammonium hydroxide, and the rest being substantially entirely water, subjecting the thus coated web to a temperature of from 250 F. to 360 F. for a residence time of from 30 seconds to 5 minutes, thereafter applying thereto a coating composition consisting essentially of from 35% to 50% by weight of photo-conductive zinc oxide particles, from 35% to 55% by weight of solvent and from 10% to 30% by weight of insulating resinous binder, and subjecting the thus coated web to a temperature of from 200 F. to 250 F. for from 30 seconds to 5 minutes to eflect removal of the solvent.

14. A process of producing electrophotographic recording members, which comprises coating a web of paper with a coating composition consisting essentially of from 1% to 3% by Weight of dimethyl-trimethylol melamine, from 5% to 10% by weight of a polyvinyl alcohol, from 5% to 10% by weight of polyvinyl acetate and the rest being substantially entirely water, subjecting the thus coated web to a temperature of from 250 F. to 360 F. for a residence time of from 30 seconds to 5 minutes, thereafter applying thereto a coating composition consisting essentially of from 35% to 50% by weight of photo-conductive zinc oxide particles, from 35 to 55% by weight of solvent and from 10% to 30% by weight of silicone resin, and subjecting the thus coated web to a temperature of from 200 F. to 250 F. for from 30 seconds to 5 minutes to effect removal of the solvent.

15. The process of producing electrophotographic recording members, which comprises, coating a web of paper with a coating composition consisting essentially of from 1% to 3% by weight of urea formaldehyde precondensate, from 2.5% to 7% by weight of a carboxylated copolymer of maleic anhydride and an ethylenically unsaturated monomeric material, from 5% to 10% by weight of polyvinyl acetate and the rest being substantially entirely water, subjecting the thus coated paper web to a temperature of from 250 F. to 360 F. for a residence time of from 30 seconds to 5 minutes, thereafter applying thereto a coating composition consisting essentially of from 35% to 50% by weight of photoconductive zinc oxide particles, from 35% to 55% by weight of solvent and from 10% to 30% by weight of silicone resin, and subjecting the thus coated web to a temperature of from 200 F. to 250 F. for from 30 seconds to 5 minutes to effect removal of the solvent.

16. A process of producing electrophotographic recording members, which comprises coating a paper web with a coating composition consisting essentially of from 1% to 35 by weight of dimethyl-trimethylol melamine, from 2.5% to 7% by weight of a copolymer of crotonic acid and vinyl acetate, from 5% to 10% by weight of polyvinyl acetate and the rest being substantially entirely water, subjecting the thus coated Web to a temperature 13 of from 250 F. to 360 F. for a residence time of from 30 seconds to 5 minutes, thereafter applying thereto a coating composition consisting essentially of from 35% to 50% by Weight of photo-conductive zinc oxide particles, from 35 to 55% by weight of toluene and from 10% to 30% by Weight of silicone resin, and subjecting the thus coated web to a temperature of from 200 F. to 250 F. for from 30 seconds to 5 minutes to effect removal of the toluene.

No references cited.

NORMAN G. TORCHIN, Primary Examiner. 

1. AN ELECTROPHOTOGRAPHIC RECORDING MEMBER CONSISTING ESSENTIALLY OF A SUBSTRATE HAVING THEREON A COATING LAYER COVERING AND BONDED TO THE FACE OF SAID SUBSTRATE, SAID COATING LAYER CONSISTING ESSENTIALLY OF THE WATERINSOLUBLE REACTION PRODUCT OF A WATER-SOLUBLE AMINOPLAST PRECONDENSATE AND A POLYMERIC FILM-FORMING MATERIAL HAVING A REACTIVE GROUP FROM THE GROUP CONSISTING OF HYDROXYL GROUPS AND CARBOXYL GROUPS, AND A TOP COATING LAYER BONDED TO SAID FIRST-MENTIONED COATING LAYER, SAID TOP COATING LAYER CONSISTING ESSENTIALLY OF PHOTO-CONDUCTOR PARTICLES UNIFORMLY DISTRIBUTED THROUGHOUT AN ELECTRICALLY INSULATING FILM-FORMING RESINOUS BINDER. 